The invention relates to a shock absorber for drilling apparatus, and more particularly to an improved shock absorber located in the drill string of a rotary drill.
In conventional blast hole drilling, a drill string formed by several interconnected sections of hollow drill shafts is both longitudinally and rotationally driven to force a drill bit into the earth. As variable subterraneous formations are encountered, the rotation of the drill string and the axial pressure applied to the drill bit generate severe vibrations or chatter which are transmitted through the drill string to the drilling machine. Such vibrations, if not dampened, may contribute significantly to fatigue failures, as exhibited by buckling of the drill string and excessive wear of machine welds, drill bits and masts. In order to combat detrimental chatter and avoid damage to the drilling apparatus, it is desirable to employ a shock absorbing coupling in the drill string having resilient means for cushioning longitudinal and torsional shocks and absorbing minor alignment variations.
One shock absorbing device of this character is disclosed in U.S. Pat. No. 3,746,330, to W. Taciuk dated July 17, 1973. In that patent, a resilient joint is provided intermediate a drive shaft and a driven shaft of a tubular drill string. The device comprises a group of resilient discs sandwiched between a series of axially spaced drive, driven and pressure plates which surround drive and driven shaft members. Pin projections extend from the driven plate into the resilient discs, while fastening means extend between the drive and pressure plates for compressing the resilient discs together and into union with the pin projections and fastening means. Longitudinal drilling forces and torque are transmitted through the resilient discs, and in addition to transmitting the necessary working forces, the resilient means must perform the function of cushioning shocks and vibrations within the drill string in both longitudinal and torsional modes.
In shock absorbers of the type described above, the resilient discs between the drive and driven plates cooperate with the hollow drive and driven shaft members to complete an air passageway which extends through the center of the tubular drill string. Pressurized air is circulated downwardly through this passageway to the drill bit, and then the air returns upwardly through the space between the drill string and drill hole to carry drill cuttings upward out of the hole.
The resilient discs of these shock absorbers have displayed limited life, making the desirability of the devices questionable. The forces and peak stresses encountered should have been well within the working limits of the material of the discs. It is a discovery of the present invention that breakdown of the discs has been due largely to the pressures and heat of the transmitted air, and thus the invention is an improvement upon the shock absorber of the above cited patent.
The internal surfaces of the resilient discs have been exposed to the pressurized air being forced through the drill stem. This pressurized air being present in substantial volume often creates interior drill string temperatures approaching 250.degree.F. It has been found that as a result of this pressure and temperature buildup, the resilient discs are susceptible to increased deterioration and subsequent material breakdown which can render the shock absorber unfit for service, and the present invention is intended to alleviate these problems.